The Future of Space Telescopes

The first big idea may revolutionize our study of terrestrial exoplanets.

The Kepler mission has determined that terrestrial planets, that is rocky planets like our Earth, are extremely common and may orbit most stars in the Milky Way.

But these planets are extremely difficult to directly image because they're dense and small and, in addition, because rock solidifies at a much higher temperature than volatiles like ammonia and water, terrestrial planets tend to form close to their parent star.

This brings a special challenge: glare.

Our sun is about 10 billion times brighter than Earth.

Train a distant telescope on us and it would be overwhelmed by the sun's rays.

So how can we find a terrestrial planet around a star light-years away? Maybe it's simple: blot out the star.

We've done this for decades with the coronagraph, a disk inside a telescope that occults a star, blocking its light so that any planets can be seen more clearly.

However, there's no such thing as a perfect shadow.

The wave nature of light causes it to bend or diffract around the edges of a coronagraph back towards the central optical axis.

This means it's never possible to completely block the star's light.

Good coronagraphs can allow detection of objects from 100,000 to a million times fainter than the central star, but nowhere near the factor of 10 billion difference between the Earth and the Sun.

In 2005, Dr. Webster Cash proposed a successor to the coronagraph, the Star Shade.

It's actually a spacecraft outfitted with thrusters to align itself between a space telescope and a star.

A Star Shade will be up to 50 meters in diameter and hover 80,000 kilometers in front of the telescope, assuming a four-meter diameter telescope mirror.

At that distance, the Star Shade acts like an artificial eclipse.

The effect of diffraction would be easier to isolate than for a typical internal coronagraph, but the Star Shade goes much further.

It will not be a simple opaque disk like a standard coronagraph.

Instead, it'll have a flower-like shape whose cleverly calculated petal geometry is designed to diffract light away from the central axis, not towards it.

The number and length of petals optimizes each Star Shade for a particular wavelength of light.

Each of those petals is articulated to fold into a very compact bud for launch, and then open up like an actual flower once in space, a bit like the James Webb Space Telescope.

The main motivation for building Star Shades is to suppress the glare of stars enough to see the planets that orbit them.

Configured right, glare is suppressed by a factor of 10 billion at 50 milliarcseconds from the star.

So one of these things would allow us to see Earth in orbit around the Sun from 60 light-years away.

There are a couple of thousand stars within that range and hundreds of Sun-like stars, many of which certainly have Earth-like planets.